The pathogenesis of allergic asthma remains unclear, however, the current understanding involves the expansion of CD4+ Th2 cells, and a breakdown in tolerance to otherwise innocuous environmental allergens (Romagnani et al. J Allergy Clin Immunol 2004; 113(3):395-400). Genetic predisposition, coupled with environmental influences appears to affect the regular suppression of Th2-mediated responses. It has been hypothesized that abnormalities in the maturation of the lung during fetal and neonatal development may render the airways more susceptible to environmental allergens, favoring polarization towards the Th2 phenotype and thus, predisposing the individual to atopy and asthma. Allergen-driven production of IL-4, IL-5 and IL-13 are typical of allergic pathologies and the secretion of such Th2-cytokines initiates isotype class-switching of B cells towards IgE, increased mucus production and recruitment of eosinophils to the airways. Since CD4+ Th2 cells represent a co-ordinating cell type in some allergies, it was suggested that the induction of counterbalancing responses might prevent the subsequent development of atopic disease. According to this modification of Strachan's hygiene hypothesis (Romagnani et al. Int Arch Allergy Immunol 1992; 98(4):279-85), microbial exposure may activate innate immune pathways that alter Th1, Th2 and Treg responses. This results in the suppression of T helper 2 cell expansion, and a consequent inhibition of isotype switching to IgE. However, several studies have suggested that viral and bacterial infections play a role in exacerbation of respiratory disease. For example, respiratory syncytial virus and Th1 inducing virulent Bordetella pertussis infection (Ennis et al. Clin Exp Allergy 2004; 34(9):1488-97) exacerbate allergic inflammation in animal models.
Gram-negative B. pertussis causes whooping cough, a severe respiratory disease responsible for significant infant morbidity and mortality worldwide. Although immunizations with either killed whole cell vaccines (Pw) or more recent acellular subunit vaccines (Pa) have had success, a re-emergence of the disease in young adults has been reported (Das P. Lancet Infect Dis 2002; 2(6):322). Typically, B. pertussis does not acutely affect this age group; however, infected adults can act as reservoirs, and increase the likelihood of infants contracting the disease prior to vaccination. Most current vaccination regimes require three doses, beginning at 2 months of age necessitating 6 months for optimal protection. Therefore, there is a need for vaccines that induce strong protection against B. pertussis in neonates.
Virulent B. pertussis infection exacerbates airway pathology in a murine model of allergen driven inflammation, despite the induction of Th1 immunity (Ennis et al. Clin Exp Allergy 2004; 34(9):1488-97). Th2 inducing Pa vaccines protect against B. pertussis-induced exacerbation of allergic asthma, but induce IL-13 both at a systemic and local level (Ennis et al. Clin Diagn Lab Immunol 2005; 12(3):409-17). In contrast, systemic immunization with Th1 inducing Pw inhibits allergic airway responsiveness (Mills et al. Dev Biol Stand. 1998; 95:31-41), suggesting that protection from allergen-driven pathology is not simply modulation of Th1/Th2 responses, but is associated with the degree of airway damage at the time of priming, such that allergen priming via the respiratory tract airways during breakdown of the airway epithelial mesenchymal unit may be a more significant factor than Th1/Th2/Treg polarization. Recently, a genetically-attenuated live vaccine against B. pertussis, BPZE1, has been developed as a candidate neonatal vaccine against whooping cough (Mielcarek et al. PLoS Pathog 2006; 2(7):e65). This live recombinant B. pertussis strain induces strong local and systemic immune responses upon intranasal delivery. Administration via the nasal route mimics natural infection and is expected to promote long-lasting immunity in children from 1 month of age (Mascart et al. J Immunology 2003; 170(1):1504-9). Three virulence factors have been targeted for attenuation; pertussis toxin, tracheal cytotoxin and dermonecrotic toxin. Using allelic exchange, genes encoding these toxins were deleted or replaced with genetically inactivated analogues in order to induce protection, without the severe pathology associated with wild-type infection. However, the influence of BPZE1 administration on third party allergen priming and allergen induced pathology is not known.